This invention concerns the polishing of the edges of semi-conductor wafers such as silicon wafers and apparatus therefor.
When silicon has been crystallised into large boules, which are generally cylindrical over most of their length, the crystalline material is sliced into thin circular discs known as silicon wafers, the diameter of each of which can be anything within the range 25-300 mm. After mounting in a vacuum chuck in an edge grinding machine, the peripheral edges of the discs are accurately ground so as to produce an accurate circular shape centered on the centre of the disc and a precisely formed edge profile, and are often notched at one position around the circular periphery.
Whereas over the majority of the area of disc the thickness is uniform and the two faces are parallel, the periphery of such discs are ground to a triangular section in which the apex of the triangle defines the outermost diameter of the wafer and is normally located midway between the two parallel faces of the disc.
In order to optimise the manufacture of semi-conductor devices using such a disc, it is important that the surface which is to be exposed to photo-lithography is damage free and in view of the microscopic size of the microcircuits which are formed on the wafers, damage near to the circumferential line between which the flat circular surface merges with the inclined surface leading to the apex, can significantly reduce the number of devices which can be made from the wafer.
In machines which do not produce a good quality ground finish to the inclined surfaces of the triangular section leading to the apex, it is necessary to etch and then polish the edges of the wafer before it can be used in the construction of semi-conductor devices. Where a good quality ground finish is produced on the inclined surfaces leading to the apex, it is only necessary to etch and polish a small quantity of material in order to obtain an undamaged edge.
It is an object of the present invention to provide an improved method and apparatus for polishing a finely ground wafer edge.
According to one aspect of the present invention there is provided a method of edge grinding and polishing circular workpieces which are transported in turn to a grinding station in which the edge of the workpiece is ground, and after grinding, the workpiece is transferred to a buffer store whilst the next workpiece is conveyed to and located in the grinding station for grinding, which further comprises the steps of rotating the workpiece in the buffer store and engaging the edge thereof with a polishing wheel, to polish its periphery whilst the next workpiece is being ground, and after polishing, the polished workpiece is removed and the said next wafer is transferred from the grinding station to the buffer store for polishing to allow a further workpiece to be located in the grinding station.
The workpieces awaiting grinding may be contained in a first store and the ground and polished workpieces may be contained in a second store. The first and second stores may be different sections of a single storage device.
The invention is particularly applicable to circular semi-conductor wafers.
The buffer store conveniently comprises a wash station and in accordance with a preferred aspect of the invention, a method of edge grinding and polishing wafers as aforesaid includes the step of washing the ground wafer before the polishing step.
The invention also lies in a method in which the wafer is washed after it has been polished.
The invention also lies in a method in which the wafer is washed both before and after the polishing step.
In accordance with a preferred feature of the invention, the method includes the step of forming and reforming a groove in the polishing wheel after one or more polishing steps have been performed.
The forming and reforming of the groove in the wheel may be effected by means of a groove-forming wheel having a triangular cross-section peripheral rim, defining in cross-section the two slanting frusto-conical edges which converge in an apex, and after polishing one or more wafers, relative movement is effected between the groove-forming wheel and the polishing wheel whilst both are rotating about their respective axes, thereby to form or reform the cross-sectional shape of the groove in the polishing wheel ready for polishing the edges of further wafers.
According to another aspect of the invention, apparatus for grinding the edges of circular wafers of semi-conductor material comprises a store within which disc-like wafers of semi-conductor material can be stored, a grinding station, means for conveying the wafers one at a time to the grinding station for effecting edge grinding thereof and after grinding conveying each wafer in turn to a buffer store and locating the wafer on a storage chuck therein, said conveying means selecting the next wafer from the store and conveying it to the grinding station to effect edge grinding thereof, a polishing wheel in the buffer store, means for rotating it and the wafer therein, means for effecting relative movement between the wafer and the polishing wheel to polish the edge of the wafer whilst the next wafer is being ground, and said conveying means serving to remove the polished wafer from the buffer store and selecting a further wafer from the store and conveying it to the grinding station for edge grinding thereof, before moving to the buffer store to recover the polished wafer from therein.
Preferably in methods and apparatus embodying the invention, a slurry is conveyed to the point of contact between the wafer and the polishing wheel so as to assist in the polishing action.
Typically the polishing wheel is compliant and is formed from a material such as urethane or serium oxide.
Typically the slurry comprises an alkali solution, preferably of pH11.
The method of the invention may preferably include the further step of effecting a rocking movement between the polishing wheel and the wafer edge by rocking either the wheel or the wafer about an axis, coincident with or parallel to a tangent which intersects the point of contact between the wafer and the polishing wheel.
In a preferred arrangement the wafer and the grooved polishing wheel are mounted for rotation about horizontally spaced apart parallel axes, the wafer is supported on a vacuum chuck and is rotated by a drive motor, the polishing wheel is driven in rotation by a second drive means, linear drive means is provided to advance and retract the polishing wheel relative to the wafer, and oscillatory drive means acts on the assembly of polishing wheel and associated drive so as to rock the wheel about a vertical axis so that the two side cheeks of the groove in the polishing wheel alternately engage the inclined edges of the edge region of the wafer.
In an alternative arrangement the wheel and its rotational drive remains stationary except for the rotation of the wheel about its axis, and linear translation means is provided for advancing the wafer and associated drive and support means towards and away from the grooved polishing wheel to engage the edge of the wafer in the groove, and oscillatory drive means serves to rock the wafer support and therefore the wafer thereon about a vertical axis so as to produce the alternating engagement between the external periphery of the wafer and the sides of the groove as the wafer and polishing wheel rotate.
References to oscillatory drive means include rocking drives and drives which rotate first in one direction and then the other about a single axis.
According to a further aspect of the invention, it is to be understood that the technique of rocking the wafer or the wheel whilst rotating so as to alternately engage the two inclined faces of the periphery of the wafer with the corresponding faces of the groove in the wheel, is not linked to an arrangement in which the wafer is mounted in a buffer store on, or associated with, an edge grinding machine. This further aspect of the invention can be incorporated into any wafer edge polishing machine in which either the wafer or the wheel can be adapted to be rocked alternately in one direction and then the other about an appropriate axis.
The invention therefore also lies in apparatus for wafer edge polishing in which there is provided means for rotating a wafer, a polishing wheel having a groove therein for accommodating the peripheral rim of the wafer, means for rotating the wheel, drive means for effecting relative movement between the wheel and the wafer to effect engagement of the wafer in the groove, and further drive means to effect relative rocking movement between the wafer and the wheel such that opposite faces of the peripheral rim of the wafer are engaged alternately by opposite regions of the surface of the groove in the wheel during the polishing process.
In any of the apparatus as aforesaid, a jet orifice is located near to the region of engagement between the wafer and the wheel, for directing a fluid slurry towards the region of engagement at least during the polishing process.
According to another feature of the invention, in any of the apparatus as aforesaid, a groove-forming wheel (a grooving tool) is mounted for engagement with the groove in the polishing wheel either by movement of the grooving tool towards the wheel or the wheel towards the tool, and for mutually rotating both wheel and tool so as to form or reform the groove in the surface of the wheel.
Where the grooving tool is a disc, the latter may be mounted to the rear of the wafer support for rotation therewith, and the forming step is performed by moving the grooved wheel into engagement with the grooving tool and rotating both so as to form or reform the groove in the wheel.
The invention also lies in any of the methods as aforesaid further comprising the step of projecting a fluid slurry towards the point of engagement between the grooved polishing wheel and the periphery of the wafer at least during polishing engagement therebetween.
The invention also lies in any of the methods as aforesaid in which a grooving tool (groove-forming wheel) is located in proximity to the grooved polishing wheel for engagement therewith to form and reform the groove in the wheel as required.
According to a further aspect of the invention, the polishing wheel has a significant axial depth, and two or more parallel grooves are formed therearound, so that as one groove becomes worn and needs reforming, the wheel can be indexed so as to use another of the grooves for polishing the edge of the wafer and the process of moving from one groove to another can be continued until all of the grooves have been utilised and need reforming before the process is arrested and a reforming process is performed on the polishing wheel grooves.
According to another aspect of the present invention, in a method of polishing the edge of a disc-like workpiece the latter is secured in position on a vacuum chuck for rotation about its centre, a polishing wheel of synthetic plastics material is brought into contact with the rotating edge of the workpiece having previously been formed around its polishing surface with a groove the cross-section of which is the complement of the edge profile of the disc, a polishing slurry is supplied to the zone of contact between the polishing wheel and the edge of the workpiece, and the polishing wheel is rotated so that relative movement exists between the polishing wheel and the disc edge.
Typically the polishing wheel is rotated contra to the direction of rotation of the workpiece in the zone of contact.
Typically the synthetics plastics material is polyurethane.
Typically the polishing slurry is colloidal silica.
Whereas with edge grinding, the grooved grinding wheel is advanced during the grinding process with a controlled feed rate, it has been found more preferable to advance a synthetic plastics polishing wheel in such a manner as to engage the edge of the workpiece in a so-called plunge mode and under constant force.
A polished surface has been obtained in a matter of a few minutes, typically of the order of 2 to 4 minutes.
The method may be performed as a modified edge grinding machine which thereby permits the disc to be edge ground and then polished while still mounted on the same chuck, by withdrawing the grinding wheel and advancing the polishing wheel and thereafter performing the aforesaid method. A washing step may be included between the grinding step an the polishing step.
Preferably the method is under the control of a computer programmed to instigate the different steps of the process.
According to another aspect of the present invention apparatus for performing the aforementioned method comprises a vacuum chuck for receiving and supporting a circular plate-like workpiece, drive means for rotating the chuck and therefore the workpiece about its centre, means for mounting a grooved polishing wheel and means for rotating same about its central axis so that its direction of rotation is contra that of the rotation of the workpiece in the region in which they will make contact, drive means for advancing and retracting the polishing wheel mounting means to enable the polishing wheel to make contact with the edge of the workpiece for polishing purposes, and means for adjusting the polishing wheel relative to the mounting means or the mounting means relative to the remainder of the apparatus, or both, to ensure that the groove in the surface of the polishing wheel accurately aligns with the edge of the workpiece, the apparatus further comprising means for supplying a polishing slurry to the zone of contact between the workpiece and the polishing wheel.
Preferably the apparatus further includes a forming wheel mounted for rotation about its central axis typically in a contra sense to the rotation of the polishing wheel where the two will come into contact, and means is provided for effecting relative movement between the polishing wheel and the forming wheel to engage the edge of the forming wheel with a surface of the polishing wheel to form a groove therein, the external periphery of the forming wheel corresponding to the external edge profile of the workpiece so that the groove in the polishing wheel corresponds to the edge profile of the workpiece.
In apparatus as aforesaid, the polishing wheel may have significant axial extent in the form of a cylinder, whereby as one grooved region thereof wears away as a result of the polishing activity, one or more further grooves can be formed at axially spaced positions therealong by appropriate indexing of either the forming wheel or the polishing wheel or both, and the apparatus is further programmed to introduce relative indexing between the forming wheel and polishing wheel and between the latter and the workpiece so that the appropriate groove in the surface of the polishing wheel is employed to polish the edge of the workpiece.
The means for supplying polishing slurry to the polishing zone preferably includes at least one nozzle for directing the slurry towards the edge of the workpiece.
Typically the nozzle or nozzles direct the slurry towards the small region of contact which exists between the polishing wheel and the workpiece.
Where the polishing wheel rotates significantly faster than the workpiece, it may assist if the slurry is directed towards the region of contact so as to be moving in the same direction as the periphery of the polishing wheel at the point of contact. Since the groove within the polishing wheel forms a small reservoir with the edge of the workpiece when the two are engaged, the rotation of the workpiece and polishing wheel may be selected so that the perimeter of the polishing wheel is moving in a downward sense at the point of contact so that the slurry can be projected down into the junction between the polishing wheel and the edge of the workpiece from above, so that a puddle of slurry is maintained in the small reservoir as aforesaid times during polishing.
Preferably the polishing action occurs within a generally closed environment so that slurry which is spattered away from the rotating parts is collected on the walls of the enclosure from which it can drain into a collection sump.
Preferably means is provided for draining slurry from the sump into a reservoir from which it can be drained or pumped for disposal or recovered for re-use.
Filtering means is preferably provided to remove particles from the slurry which are greater than a given size if the recovered slurry is to be reused.
Preferably means is provided for sensing the flow of slurry through the nozzle which includes an interlock for retracting the polishing wheel to disengage it from the workpiece in the event that the slurry flow ceases or drops below a predetermined flow rate.
In apparatus as aforesaid, where the polishing wheel is polyurethane, a preferred slurry is colloidal silica.
A preferred material for the polishing wheel is polyurethane.
The apparatus as aforesaid may comprise a wafer grinding machine in which one or more grinding wheels are advanced under computer control into engagement with the workpiece to effect an initial (or finish) grinding of the edge, and on which machine the polishing wheel is also mounted, and the polishing wheel again under computer control, is advanced into contact with the edge of the workpiece after the grinding process has been completed, to perform a polishing stage.
Where the polishing wheel is located on the grinding machine, the grinding wheel and polishing wheel may be mounted on the same wheelhead assembly for rotation by a common drive means such as an electric, hydraulic or pneumatic motor.
Where the same drive is used, the speed of rotation may be varied as between grinding and polishing as appropriate.
Furthermore, where the same advance and retract mechanism is used for advancing and retracting the grinding wheel and polishing wheel, the computer is further programmed to alter the feed characteristics from when the grinding wheel is advanced, to a constant force plunge mode for when the polishing wheel is to be used.
Since the grinding process will produce a considerable quantity of swarf and spatter during the grinding process, the grinding process is preferably carried out within a similar enclosure as is the preferred arrangement for the polishing step, and where the same enclosure is used, preferably a washing step is included between the grinding and polishing steps so as to wash the interior of the enclosure and more particularly at least the surface of the polishing wheel, to remove any traces of grinding swarf, before the polishing step is performed.
In addition or instead, a protective sleeve or housing may be provided for covering the polishing wheel except when the latter is to be made available for polishing, so that there is little or no chance of any grinding swarf finding its way onto the surface of the polishing wheel.
Where a sleeve or housing shrouds the polishing wheel during the grinding step, the shrouding enclosure is preferably openable and movable under control of the computer after the grinding process has ceased and any washing step has been performed to expose the polishing wheel and allow the latter to move into its polishing position.